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The attack on the National Republican Congressional Committee (NRCC), the campaign arm for Republicans in the House of Representatives, left the email accounts of high ranking members compromised for several months before the hack was detected. The FBI were informed days later and launched an investigation into the source of the cyber attack, the NRCC said.

The email accounts of four "senior staffers" were breached and involved "multiple intrusions over a few months" before the attack was discovered in April, an official connected to the investigation told the Telegraph.

The attack, first reported by the website Politico, was deemed so sensitive that senior Republicans, including the House Speaker Paul Ryan, were not told of the breach until reporters began inquiring about it on Monday.

The committee took on the services of prominent Washington legal firms as well as the public strategy firm Mercury to handle the incident.

A spokesman for the committee confirmed the hack on Tuesday. “The NRCC can confirm that it was the victim of a cyber intrusion by an unknown entity,” the spokesman said.

“The cybersecurity of the Committee’s data is paramount, and upon learning of the intrusion, the NRCC immediately launched an internal investigation and notified the FBI, which is now investigating the matter."

He added that the NRCC would not be commenting further "to protect the integrity of that investigation".